Method for installing cathodic protection

ABSTRACT

A method for providing cathodic protection for a metallic structure in contact with the ground, for example, a pipeline, comprises disposing a suitable anode at depth in a borehole so that the anode is disposed generally opposite a preselected ground zone or stratum of relatively low electrical resistivity. A liquid electrolyte which is operatively compatible with both the surrounding ground and with the anode is provided in the borehole and may comprise naturally occurring ground water which may optionally be modified, for example, by the addition of an ionizable salt.

United States atent 2,244,322 6/1941 Zoller et a1. 204/148 2,360,244 10/1944 McAnnent 204/147 2,803,602 8/1957 DeCowsky et al. 204/196 2,851,413 9/1958 Hosford 204/196 3,075,911 l/l963 Anderson 204/196 OTHER REFERENCES Underground Corrosion NBS Circular No. 579, 1957. pp. 186- I88 Primary ExaminerT. Tung Attorneys-Arne l. Fors and Frank I. Piper ABSTRACT: A method for providing cathodic protection for a metallic structure in contact with the ground, for example, a pipeline, comprises disposing a suitable anode at depth in a borehole so that the anode is disposed generally opposite a ,preselected ground zone or stratum of relatively low electrical resistivity. A liquid electrolyte which is operatively compatible with both the surrounding ground and with the anode is provided in the borehole and may comprise naturally occurring ground water which may optionally be modified, for example, by the addition of an ionizable salt.

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; LENSES RESISIIVITY OHM-CNS 1 METHOD FOR INSTALLING CATHODIC PROTECTION BACKGROUND OF THE INVENTION The present invention relates to a method for providing cathodic protection for metallic structures in contact with the ground and to installations comprising structures protected by such a method.

The corrosion of under ground structures such as pipelines and steel substructures and of other metallic structures in contact with the ground is a very serious and extensive problem and numerous proposals have been made for the provision of cathodic protection for such structures to prevent electrochemical corrosion thereof.

Existing procedures for providing such cathodic protection can conveniently be classified into two types. The first type involves the use of a sacrificial anode buried in the ground some distance from the structure to be protected. Such sacrificial anodes are formed of metals such as magnesium and zinc which are more electronegative than the structures being protected.

Such sacrificial anodes are consumed relatively rapidly by electrochemical reaction while corrosion of the metallic structure such as a pipeline is prevented. Since such sacrificial anodes must be renewed relatively frequently, it is generally considered to be totally uneconomical to dispose such anodes at depth in the ground even though their disposition near the surface of the ground, for example at depths down to 30 feet, can present some relatively serious problems which are fully recognized by those conversant with corrosion technology and engineering. The relatively high operating and replacement costs involved in the use of a necessarily large number of large sacrificial anodes to provide adequate protection has led to the use of so-called permanent or impressed voltage-type anodes. In this latter procedure, a voltage is externally impressed from a suitable source of direct electrical current between the structure to be protected and an anode buried in the ground so as to ensure that the flow of current between the anode and the structure through the ground is always such that the structure forms the cathode and its corrosion is consequently prevented.

Since, in this second procedure, reliance is place on the externally impressed current, it is not necessary to use a highly electronegative anode. Consequently, the anodes which are presently used in this system are normally formed from electropositive metals or from graphite presenting the best possible balance between such factors as capital cost, maintenance cost, operating effectiveness and anode life. Merely, by way of illustration there may be mentioned the use in this second procedure of anodes formed of graphite, high silicon cast iron, lead-silver alloys and lead-platinum anodes as well as of anodes comprising relatively thin platinum coatings on cores formed of metals such as titanium, tantalum and niobium.

As already indicated, when corrosion-protection anodes are disposed close to the surface of the ground, their operation is frequently relatively unreliable and this is as true in the case of the permanent-type anodes as it is in the case of the aforementioned sacrificial anodes. For example, the anodes may be adversely affected both directly and in their protection perfonnance by such factors as variations in the level and composition of surface water ground pollution resulting from the presence of industrial effluent, freezing and numerous other factors. Another important problem which arises when a corrosion-protecting anode is disposed near the surface of the ground is that the electrical current flow path between such an anode and the structure being protected is relatively direct and consequently a large number of relatively closely spaced anodes is needed if a large structure such as a pipeline is to be protected adequately.

It is already well known that the necessary interanode spacing for the protection of large structures such as pipelines can be considerably reduced by disposing the anodes at greater depths, such as 30 feet or more, in the ground. With such presently known deep anode systems, the current flows from the deeply buried permanent anode into the surrounding stratum and then upwardly to the structure to be protected. Generally such upward current flow will involve the passage of the current through strata of higher electrical resistivity than that immediately surrounding the anode, and this interposition of a higher resistivity stratum is beneficial in extending the distance or range over which cathodic protection is provided. Consequently, in the case of pipelines, the boreholes containing the anodes can be spaced apart along the line at greater separations.

In order to provide effective electrical coupling between such a deep anode and the surrounding strata, it is customary to provide a solid backfill above the anode in the borehole. For example, the use of coke breeze for this purpose is very widespread. Numerous proposals have been made for improving the corrosion protection provided by such systems, and for reducing the capital and operating costs of such systems. The attainment of effective protection on a desired economic basis is still, however, plagued by numerous difficulties.

The current outputs of such permanent anodes with solid backfill will in some cases progessively decrease, sometimes rapidly and sometimes somewhat gradually and, since the protection afforded by such a system is dependent on the anodestructure current flow, the protection provided by such a system will inevitably decrease when the current flow decreases in this manner. When such a situation arises, as it might after even only a few weeks operation, there is no alternative but to install additional or new anodes or to resort to the use of higher and higher anode voltages. In order to reduce the need for frequent replacement of such deep well anodes and in order to obtain adequate flow of protective current to the structure being protected, it has become customary to dispose a relatively long string of such anodes in each such borehole. This is, of course, very costly.

It is a principal object of the present invention to provide a method for providing cathodic protection for a metallic structure in contact with the ground which method is characterized by giving improved protection as indicated by an economic analysis of its operation.

Another object of this invention is to provide a novel method for providing cathodic protection by the use of which method operational utilization of a single anode or of a set of anodes in a single borehole may be significantly prolonged.

Yet another object of this invention is to provide a novel method for providing cathodic protection in which method steps may be taken at any time after installation of the anode or anodes in the borehole to inspect, rejuvenate or otherwise treat the installation to avoid, counteract or otherwise modify detrimental conditions that develop during its operation.

Another object of this invention is the provision ofa method for providing cathodic protection by which method improved anode-structure current flow may be obtained without the use of particularly long strings of anodes in each borehole.

A further object of this invention is to provide a method for providing cathodic protection by means of which the anodes which are utilized can readily be removed from the boreholes, and in which method any undesirable materials formed in such a borehole during operation of the system can be removed relatively facilely therefrom.

Another important object of this invention is to provide an installation comprising a metallic structure in contact with the ground which structure is cathodically protected against corrosion by the method of the invention.

Other objects will become apparent as the description herein proceeds.

SUMMARY OF THE INVENTION The present invention is based on the finding that, by the use of the aforementioned permanent or impressed voltagetype of anode in a deep well bore together with the presence of an aqueous liquid electrolyte meeting certain requirements around such an anode in the bore, it is possible to obtain exceptionally efl'ective corrosion protection for a metallic structure in contact with the ground provided that the electrolyte and anode which are used and the position of such an anode in the borehole comply with certain critical parameters which will be explained in greater detail hereinafter.

In its broadest scope, the present invention provides a method for providing cathodic protection for a metallic structure in contact with the ground, which method comprises providing a borehole in the ground, which borehole extends downwardly through a plurality of ground zones of different electrical resistivities, identifying a ground zone of relatively low electrical resistivity at depth within said borehole, ensuring the presence within said borehole of an aqueous liquid electrolyte compatible with said ground, positioning an anode in said electrolyte in said borehole generally opposite said ground zone of relatively low electrical resistivity, said anode being operatively compatible with said electrolyte, and connecting said metallic structure and said anode to a source of direct electrical current whereby a positive voltage is applied to said anode with respect to said metallic structure.

In order to facilitate comprehension of the various factors which determine the method of carrying out this invention, reference will first be made to the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING The accompanying drawing is a somewhat schematic illustration of a cathodic protection installation in accordance with the invention and also shows a typical resistivity log as obtained during the carrying out the method of the invention and as used for determining the correct positions for the anode or anodes in the borehole.

DESCRIPTION OF THE PREFERRED EMBODIMENTS There is shown somewhat schematically in the accompanying drawing a metallic structure such as a pipeline which is to be protected in accordance with the invention against corrosion. Such corrosion can be caused for example, by stray currents passing through the ground 11. For convenience, the description hereinafter will be restricted to the protection of such a pipeline but is should be understood that the invention is widely applicable to the protection of other metallic structures which are in contact with the ground and which may, for example, be partially or completely buried in the ground.

The cathodic protection installation shown comprises a borehole generally indicated at 12 which extends downwardly through various ground strata or zones which are shown as comprising ground, strata 13, rock strata l4 and substrata 15. Disposed generally axially within the borehole 12 below the level of the rock strata 13, there are supported a pair of impressed voltage-type anodes which are shown schematically and diametrically enlarge at 16 and 17.

The anodes l6 and 17 are interconnected by an insulated electrical conductor or cable 18 while an insulated conductor or cable 19 connects the upper anode 16 to a suitable source 20 of direct electrical current. Such a source 20 is conventional and may, for example, comprise a number of storage batteries or more usually a rectifier supply unit. Since such a source is conventional, it will not be described herein greater detail. It will be understood that in the installation shown, the cable 19 also serves to support the anodes 16 and 17 at the desired depths in the bore hole 12 and that this cable 19 will be suitably insulated. Separate anode suspension members may, of course, be used.

It will also be noted that in accordance with a preferred feature of this invention, the borehole 12 is provided with a liner 22 which is formed of an electrically insulating material. An important function of this liner 22 is to prevent plugging of the bore hole by collapse of its walls. The lower portion of the liner 22 is perforated as shown at 23 to permit ion flow therethrough and it will be noted that such perforations extend from the bottom of the liner 22 to a level above the upper anode 16. The installation shown is essentially completed by a cap 24 and a vent pipe 25, the purpose of which will become apparent as the description herein proceeds.

It should also be noted that an aqueous electrolyte 26 is provided within the borehole 12. The nature of this electrolyte and its purpose will be described hereinafter in greater detail but, before proceeding with such description, it should perhaps be stressed that the installation herein described with reference to the accompanying drawing differs from known deep anode systems for providing cathodic protection in that it involves the use of permanent (or impressed voltage-type) anodes disposed in a deep borehole, such anodes being surrounded by an aqueous electrolyte. As already explained, this differs from the known use of deep well anodes in which it has been standard practice to provide a solid backfill such as coke breeze around and above the anodes in an attempt to obtain effective electrical coupling and larger anode area between the anodes and the surrounding strata.

In order to obtain optimum electrical coupling for the anode, or each anode, the positioning of the anodes within the borehole is carried out in accordance with a very important feature of this invention so that each such anode is positioned generally opposite a ground zone or stratum of relatively low electrical resistivity. The second step, therefore, in the method of the invention after having obtained or drilled a suitable borehole in the ground involves the identification of one or more suitable low resistivity ground zones. Any appropriate resistivity logging method may be used for this purpose and a typical resistivity log obtained for this purpose is illustrated in the accompanying drawing. From this log, it will be seen that the strata resistivities for the particular borehole increase gradually downwardly from the ground surface through the ground strata 13 to a depth of about feet. Between about 80 and about 110 feet, there exists in the rock strata 14 a very pronounced low resistivity ground zone while the substrata 15 extending between the and the 130 foot depths show somewhat higher resistivities. Below a depth of about l35 feet, the resistivity varies only slightly about a relatively low average value.

In this particular instance in order to obtain optimum anode-ground coupling, i.e. maximum current flow from the anode into the ground for a given voltage applied to the anode, such an anode will be placed in accordance with this invention at a depth of from about 90 to feet. A l5foot anode 16 (as shown) could conveniently be used at this position. If desired, for example, to obtain additional current flow through the ground to the pipeline 10, further anodes, such as anode 17 having a length of 20 feet, may be positioned at depths greater than about US feet.

In general, it is preferred to place the, or each, anode generally opposite a relatively low resistivity ground zone which is itself disposed below a ground zone of higher resistivity. As already indicated, such anode disposition will lead to protection of the metallic structure such as the pipeline 10 to a greater distance or range from the borehole 12.

The length of each of the anodes and the number of anodes to be used in each borehole such as borehole 12 in the method of this invention, will be determined by several factors, amongst which there may be mentioned the primary requirement of establishment of an adequate anode-structure current flow to give the necessary corrosion protection. By placing the anodes opposite preselected ground zones of relatively low electrical resistivities and by conforming with other important requirements of the invention still to be explained, it is, however, possible to obtain effective protective current flow without resorting to the use of an excessive number of anodes or to the use of lengthy anode strings as is generally the case with the existing method in which solid backfills are used.

As hereinbefore indicated, the present invention is restricted to cathodic protection methods and installations in which permanent anodes are disposed at depth in a borehole. Although it is somewhat difficult to provide a precise definition for the phrase at depth when used herein and in the appended claims, this phrase is intended to make it clear that the invention involves disposing the anodes at depths similar to those used in the well-known deep anode system. Such systems can perhaps best be distinguished from the surface anode type by the use of a somewhat negative-type definition. The use of sacrificial anode as the sole source of protective anode-structure current is precluded from the scope of this invention since the latter involves the positioning of anodes at such depths that, if in fact only sacrificial anodes were used, the total anode area, i.e. the anode size and/or the number of anodes, required would be impractically high so that such use would be economically infeasible.

In general, the anodes used in the method of the present invention will be disposed at depths of at least feet and more particularly at depths of more than about 30 feet. Under the majority of practical circumstances, it will be economically infeasible to position anodes at depths greater than 500 feet since at such depths the additional cost for drilling the borehole becomes too high. in such a case, increased anode area would frequently also be required to provide adequate anode-structure current flow and the use of such great depths is, therefore, generally contra-indicated. Nevertheless, it is possible that under somewhat unusual circumstances, it could be desirable to use anodes at such great depths. From an economic and practical point of view, the anodes used in the method of the present invention will normally be disposed at depths in the range of from about 50 to about 150 feet.

Another important feature of the method of the invention involves the step of ensuring the presence within said borehole of an aqueous liquid electrolyte. Normally, an aqueous solution will be present in the borehole due to the naturally occurring ground water existing in the ground through which the borehole is drilled. This naturally occurring ground water may itself constitute the aqueous electrolyte required in accordance with the method of the invention. Alternatively, such naturally occurring ground water may be chemically modified to provide some effective operation as will be explained hereinafter in greater detail. In the relatively rare event that no naturally occurring ground water is present in the borehole, a separately prepared liquid electrolyte may be introduced thereinto to provide the required liquid environment for the anode. Such electrolyte will be chosen so as to be operatively compatible with the electrolyte naturally present in bound form in the surrounding ground zones as identified by analyzing samples thereof. In such a case, it will be necessary to seal any porous strata below the anodes by using a suitable liner to prevent drainage of the electrolyte from the borehole. In order to determine the most suitable and practical liquid electrolyte to be used in the method of the invention, attention must be given to such factors as the chemical nature of the surrounding ground zones, the chemical nature of any naturally occurring ground water present in the borehole, the chemical nature of the anode or anodes being used, and the chemical resistance of the electrical insulating material ofthe cables 18 and 19.

As hereinbefore indicated, these factors are set down as essential features of this invention by requiring that the aqueous electrolyte is operatively compatible with the surrounding ground and that the anode, or each anode, is operatively compatible with the aqueous electrolyte. As was the case with the phrase at depth" hereinbefore discussed, it is difficult to set down a general definition of the phrase "operatively compatible which will apply to all possible combinations of the several variables.

In general it can, however, be indicated that the selection of a suitable operatively compatible aqueous liquid electrolyte and of a suitable anode material will be determined on the basis of the chemical composition of any naturally occurring ground water present in the borehole. The material of the anode will be such that the anode will operate effectively in the presence of such ground water. For example, if such ground water contains a relatively large proportion of chloride ions, an anode will be used which is sufficiently resistant to attack by chloride ions or by chlorine under the actual operating conditions. In such a case, the use of an unmodified high silicon cast iron anode would be completely contra-indicated. Similarly, if a naturally occurring ground water present in the borehole contained a significant proportion of sulfate ions, anodes of materials which cannot be used with such electrolytes, for example, graphite and cast iron anodes, would not be the preferred materials from the point of view of anode performance. When naturally occurring ground water is present in the borehole, such water will be sampled and analyzed to provide data on which the selection of a suitable anode material can then be based. Another important factor affecting the choice of a suitable anode material is the anode-electrolyte potential drop to which the anode will be subjected in use. The permissible maximum value for such potential drops for different anode materials with which an oxide or other protective film must be maintained on the anode or on its supporting core are known or are readily determinable for the more common permanent anode materials and these values will not be exceeded during operation of the method in accordance with the invention using this particular type of anode. The values of the actual anode-electrolyte potential drops are, however, also dependent on the voltage applied to the anode and this is in practice in turn dependent on the anode-structure current which is required so as to give adequate corrosion protection, a higher anode-structure cur rent requiring a higher anode voltage and consequently, with other conditions unchanged, a higher anode-electrolyte potential drop. in view of the operating limits on such potential drops, it is necessary with the conventional solid backfilled systems to resort in such cases to the use of larger and greater numbers of anodes to obtain adequate anode-structure current.

An important feature of the present invention is that it permits ready and continuous access to the electrolyte from the top of the borehole so that this electrolyte can be chemically modified if and when required to improve the anode-electrolyte interaction, for example, to reduce the anode-electrolyte potential drop thereby frequently avoiding the need for the use of larger or more anodes as required in solid backfilled installations to ensure continuing effective performance.

One particularly important manner in which the aqueous electrolyte may be modified to improve the anode-electrolyte interaction is by the addition thereto of an ionizable salt. Such addition can be very effective in reducing the anode-electrolyte potential drop and may incidentally improve the electrolyte conductivity thereby permitting greater current flow without requiring the use of higher anode voltages or areas. Any such addition of an ionizable salt to the electrolyte in this manner will be governed by the chemical nature of the existing electrolyte. Any such salt which is added must be chemically compatible with the surrounding ground, i.e. there must be no undesirable chemical interaction therebetween, and such added salt must have no adverse effect on anode behaviour, for example, it must not prevent the formation of any protective oxide films which are required on the metal anodes or their supporting cores for effective noncorroding anode operation. In many cases and particularly where chloride ions are present in naturally occurring ground water, a water-soluble chloride such as sodium chloride or potassium chloride can frequently be added with beneficial results to such an electrolyte comprising a naturally occurring ground water.

Other electrolyte additives which may be mentioned by way of further example are inhibitors which will serve to reduce anode corrosion, sequestering or other agents which will inhibit the formation of undesirable precipitates on the anode and additives which will facilitate the liberation of any gaseous products which are released at the anode.

The expression operatively compatible" when applied to the electrolyte and to the anode will be understood by those conversant with corrosion technology. An important distinction should, however, be drawn between this statement that the meaning of the expression will be obvious to those skilled in the art and the claims that the specific use of electrolytes and anodes which are operatively compatible with each other and that the electrolyte will be compatible with the surrounding ground is both novel and inventive. The distinction is fine but exceedingly important. The specific selection of electrolyte/anode combinations with the nature of the surrounding ground in mind is a completely novel step in the installation of deep well permanent anode cathodic protection systems and, in addition to such novelty, when considered with the other important novel features of the invention, leads to very significant operating advantages which will be identified in greater detail hereinafter and which cannot be obtained with the conventional solid back-filled systems.

Another important feature concerning the chemical nature of the electrolyte is that is should be chemically nonreactive with respect to both the liner member 22 and the insulation on the cables 18 and 20. Again numerous factors come into play. There may, for example, be mentioned the costs of the different materials available for such linings and insulation, and the chemical nature of any naturally occurring ground water electrolyte. The properties of the selected materials will in turn apply certain restrictions to what materials can be added to the electrolyte.

The more important advantages presented by the method of the invention will now be summarized, reference also being made at the same time to some other important features of the invention.

One very important advantage of this invention is that it pennits the same degree of corrosion protection to be obtained as with conventional solid back-filled anodes but with the use of smaller anodes or with the use of a smaller number of anodes. For example, in one particular bore hole, generally equivalent initial anode-structure currents were obtained by the use of two anodes, one 20 -feet long and the other -feet long, in a naturally occurring ground water electrolyte with the small addition of sodium chloride, on the one hand, and by the use of a 120 -foot anode string using a solid coke breeze backfill. The advantage results primarily from the improved anode-ground coupling resulting partially from the disposition of the anodes in optimum positions with reference to the ground zones and partially from the considerably improved contact obtained between the ground and the anode due to the interposition of a continuous liquid electrolyte of good conductivity in distinction to that obtained with the solid backfill in which interparticle contact can become highly resistive due to the reactions taking place therein.

Consequently, the capital cost of the anodes is lower and the use of such smaller anodes permits such anodes to be disposed in optimum positions more readily. In turn, this leads to greater utilization of the increased protection range obtainable by making use of the spreading effect of an overlying high resistivity ground zone as already considered herein.

Another important advantage of this invention is that the progressive loss of anode-structure current during use which occurs all too frequently with the conventional solid backfilled systems can be avoided or, it if does occur, can be rectified in a relatively simple manner. The illustrative example of the use of a 120 -foot anode string mentioned above illustrates this point. in use, the current obtained from that string decreased progressively to such an extent that the anodes had to be abandoned after about a year of operation whereas the installations of this invention as hereinbefore exemplified by the two-anode arrangement functioned successfully for several years with insignificant loss of anode-structure current. Loss of anode-structure current flow due to the entrapment of gaseous product bubbles around the anode is one cause of loss of protective current flow in the existing solid back-filled systems. This problem does not arise with the method of this invention since, due to the presence of a liquid environment around the anode, such bubbles are free to escape upwardly for venting to the atmosphere through the vent pipe 25. Similarly solid precipitates formed during operation are fee to fall to the bottom of the borehole as shown at 27 in the drawing. In solid back-filled systems, such precipitates frequently contribute to anode deactivation. This advantageous difference is particularly important under circumstances under which siliceous precipitates are likely to be formed. In accordance with the invention, such solid precipitates may readily be removed from the bottom of the borehole 12 from time to time as required or the electrolyte may be withdrawn, filtered and returned on a continuous or semicontinuous basis for this same purpose. This is not possible, however, with a solid backfilled system where all the backfill material would need first to be removed at considerable expense.

Another feature made possible by the absence of solid backfill material is that the liquid electrolyte may be periodically removed from the borehole and treated outside the hole. For, example, with many systems, the acidity of the electrolyte will progressively increase during use of the system and such increased acidity will in turn lead to increased anode corrosiion and attack on the liner 22 and on the cable insulation. In accordance with a useful feature of the invention, it is a simple matter to withdraw sucli'acidified electrolyte and to neutralize it before returning it to the borehole so avoiding the need for the use of expensive highly acid-resistant materials for the borehole liner and for the cable insulation. Any precipitates formed during such neutralization can, of course, be separated, for example, by filtration before such electrolyte is returned to the borehole, if necessary. Yet another possible use for such periodic treatment of the electrolyte is that changes taking place in the electrolyte during use which might subsequently lead to modification or breakdown of the surrounding ground zone and the consequent drop in anodestructure current can be rectified as required. Such modification, if not avoided, may lead to the conversion of a relatively low resistivity ground zone into an exceptionally high resistivity material; this effect, which is not fully understood, has been referred to in certain literature as induration. This difficulty is readily avoided in the method of the invention by treating the electrolyte as required.

Yet another advantage of the method of the invention is that the anodes can be repositioned in boreholes at any time if so desired and similarly can be removed for inspection and/or maintenance when desired.

With the use of two or more anodes in a single borehole, such anodes can obviously be operated independently by the provision of separate feed conductors or they can be connected together as shown in FIG. 1 and then operated as a single anode.

What I claim as new and desire to protect by Letters Patent of the United States is:

1. A method for providing cathodic protection for a metallic structure in contact with the ground, which method comprises providing a borehole in the ground, which borehole extends downwardly through a plurality of ground zones of different electrical resistivities, identifying a ground zone of relatively low electrical resistivity at a depth of at least 20 feet within said borehole, ensuring the presence within said borehole of an aqueous liquid electrolyte of a water-soluble chloride, positioning an anode freely in said electrolyte in said borehole generally opposite said ground zone of relatively low electrical resistivity, and connecting said metallic structure and said anode to a source of direct electrical current whereby a positive voltage is applied to said anode with respect to said metallic structure.

2. A method as claimed in claim 1 wherein said electrolyte comprises naturally occurring ground water.

3. A method as claimed in claim 2 which comprises the step of sampling and analyzing said naturally occurring ground water.

Q. A method as claimed in claim 3 which additionally comprises the step of chemically modifying said naturally occurring ground water by the addition of an ionizsble salt of a water-soluble chloride.

5. A method claimed in claim 1 which additionally comprises periodically removing said electrolyte from said borehole for modification thereof, and returning a resulting modified electrolyte to said borehole.

6. A method as claimed in claim 1 in which said anode is disposed within said borehole upwardly of a lower end thereof, whereby solid materials formed during the flow of electrical current through said electrolyte settle below said anode.

7. A method as claimed in claim 6 in which said solid materials are periodically removed from said borehole.

8. A method as claimed in claim I which comprises disposing a plurality of anodes in said electrolyte in said borehole opposite respective ground zones of relatively low electrical resistivities.

9. A method as claimed in claim 1 which comprises conmeeting said anode to said source of direct electrical current by means of an electrical conductor having an external covering of an electrically insulting material which is selected so as to be substantially chemically nonreactive with said electrolyte 10. A method as claimed in claim 1 in which said anode is selected from the group consisting of graphite, high silicon iron alloys, lead-silver alloys, lead-platinum alloys, platinizedtitanium, platinized-tantalum and platinized -niobium.

11. A method as claimed in claim 1 which comprises disposing said anode at a depth in said borehole of from 50 to 500 feet.

12. A method as claimed in claim 1 which comprises disposing a porous liner in said borehole, which liner is formed of a substantially chemically nonreactive material. 

2. A method as claimed in claim 1 wherein said electrolyte comprises naturally occurring ground water.
 3. A method as claimed in claim 2 which comprises the step of sampling and analyzing said naturally occurring ground water.
 4. A method as claimed in claim 3 which additionally comprises the step of chemically modifying said naturally occurring ground water by the addition of an ionizable salt of a water-soluble chloride.
 5. A method claimed in claim 1 which additionally comprises periodically removing said electrolyte from said borehole for modification thereof, and returning a resulting modified electrolyte to said borehole.
 6. A method as claimed in claim 1 in which said anode is disposed within said borehole upwardly of a lower end thereof, whereby solid materials formed during the flow of electrical current through said electrolyte settle below said anode.
 7. A method as claimed in claim 6 in which said solid materials are periodically removed from said borehole.
 8. A method as claimed in claim 1 which comprises disposing a plurality of anodes in said electrolyte in said borehole opposite respective ground zones of relatively low electrical resisTivities.
 9. A method as claimed in claim 1 which comprises connecting said anode to said source of direct electrical current by means of an electrical conductor having an external covering of an electrically insulting material which is selected so as to be substantially chemically nonreactive with said electrolyte.
 10. A method as claimed in claim 1 in which said anode is selected from the group consisting of graphite, high silicon iron alloys, lead-silver alloys, lead-platinum alloys, platinized-titanium, platinized-tantalum and platinized-niobium.
 11. A method as claimed in claim 1 which comprises disposing said anode at a depth in said borehole of from 50 to 500 feet.
 12. A method as claimed in claim 1 which comprises disposing a porous liner in said borehole, which liner is formed of a substantially chemically nonreactive material. 